Amphoteric metal electroplating processes



AMPI-IOTERIC METAL ELECTROPLATING PROCESSES George Chandler Cox,Charleston, W. Va.

No Drawing. Application October 17, 1957 Serial No. 690,640

6 Claims. (Cl. 204- 38) As far as is known no method has been developedfor electroplating a metal object except when the object is immersed inor is in contact with a solution of a salt of the metal which it isdesired to electroplate.

This invention relates to improved methods and processes forelectrodepositing certain metals without the salt of the amphotericmetal being deposited.

An object of this invention is to chemically react a calcareous coatingon a metal structure, similar to the types produced by the methods givenin my US. Patents No. 2,200,469 of May 14, 1940, or No. 2,534,234 ofDecember 19, 1950, with a soluble salt of an amphoteric metal so that aninsoluble salt or basic complex containing the amphoteric metal will bedeposited either in or in place of the calcareous coating on the metalstructure, and thereafter suitably electrolyzing the complex to give thedesired amphoteric metal coatings.

An object is to chemically react a low soluble basic non-metalliccoating on a metal structure with a soluble salt of an amphoteric metalin order to produce a low soluble salt or basic complex containing theamphoteric metal in or in place of the basic low soluble coating andthereafter suitably electrolyze the complex to give the desiredamphoteric metal coating.

A further object is to cause a basic low soluble inorganic non-metalliccoating which has been sprayed in place on a metal structure, applied asa slurry on the structure or painted on the structure to chemicallyreact with a soluble salt of an amphoteric metal thereby producing insitu a low soluble salt or basic complex containing the amphotericmetal, and thereafter suitably electrolyzing the complex to give thedesired amphoteric metal deposit.

A further object is to combine into an amphoteric coating complex typeof slurry a low soluble salt of an amphoteric metal with other lowsoluble coating constituents and then apply this material to a metalstructure by spraying, painting or dipping, and thereafter suitablyelectrolyzing this amphoteric metal coating complex to form a metaldeposit of the amphoteric metal on the metal structure.

For simplicity the chemical reaction product containing the low solubleamphoteric metal complex, which can be produced in situ and whichresults from the reaction of a basic, low soluble, inorganicnon-metallic coating with a soluble salt of an amphoteric metal, orwhich results from a mechanical mixture of a low soluble salt of anamphoteric metal with the other low soluble coating constituents in theform of a slurry, will hereafter be referred to as the amphotericcoating complex.

2,919,233 Patented Dec. 29," 1959 The originally deposited basic, lowsoluble, inorganic, non-metallic coating which can be deposited on thestructure to be protected by electrolytic means, when using the methodsof the above-mentioned patents, or by mechanical spraying, dipping intoa slurry, painting, or by other means will hereafter, for simplicity, bereferred to as the initial basic low soluble coating deposit.

A further object is to solubilize increment after increment byelectrolytic means the amphoteric coating complex at the interfacebetween this coating complex and the metal on which it is deposited,while at the same time electrolytically depositing the desiredamphoteric metal on the cathode surface to be electroplated from thesolubilized product.

In other words, these processes allow an amphoteric metal: (1) to beincorporated by chemical or mechanical means in a low soluble form inplace of or in a basic low soluble protective coating on a metalstructure; (2) to be slowly solubilized as needed by an electric currentat the cathode surface of the structure; and (3) then to beelectrolytically deposited in metallic form on the cathode surface ofthe structure from the solubilized product.

Step (1) of the preceding paragraph can be omitted by combining asuitable low soluble amphoteric metal complex directly into a slurrywhich can be sprayed or painted in place on the structure, or intowhichthe structure to be coated can be dipped. Steps (2) and (3) aslisted in the preceding paragraph would then be carried out as given.

Another object is to decrease the cost of electrolytically depositing ametallic coating of an amphoteric metal which can be electrodeposited ona structure such as the inner surface of a large tank, etc. For example,when it is desired to electroplate the inside of a large tank with anyone of the amphoteric metals which can be electrodeposited, a fewgallons of the desired concentration of the amphoteric metal salt wouldbe sprayed on to the initial basic coating deposit and then steps (2)and (3) as listed above would be carried out. It is evident that thesesteps (2) and (3) can be carried out when using a comparativelyinexpensive electrolyte, such, for example, as a solution of sodiumchloride or sulphate or similar potassium salts. Of course, for theselatter two steps any electrolyte could be used which would liberate astrong concentration of sodium or potassium hydroxide or other strongbase on the cathode surface.

When the ordinary electroplating methods are used to deposit a metalliccoating on the inside walls, for example, of a large tank or tank shipcompartment, the

cost of the metal salt required to fill the compartment or tank with asuitable electrolyte is generally prohibitive. These processes ofapplicant allow large tanks or other large structures which can beplaced in tanks to be electroplated with lead, zinc or tin at acomparatively low cost.

Similarly the wind-water areas of steel piling'or bulkheads in sea watercan be electroplated with lead, tin or zinc at comparatively low cost.

Lead, zinc or tin are the commercially economic metals which will: (a)form a low soluble compound with a limited amount of alkali, (b) withfurther additions of alkali become soluble, and then (0) can beelectrolytically reduced to the metallic form and deposited on thecathode surface.

Lead coatings When, for example, a calcareous coating of 'theapproximate composition given in my U.S. Patent No.

a reaction similar to one or more of the following will take place:

' Pb (N +Ca(OH) Pb(OH) +Ca(NO 2 Pb a)2+ Pb )2+ 3)2 If a limited amountof sodium hydroxide is present then:

Pb (N0 -E-2NaOH- Pb (OH) +2NaNO Therefore, when an initial low solublebasic coating deposit is treated with a soluble lead salt such as leadnitrate some of the low soluble lead hydroxide will be chemically formedand deposited in situ in the basic coating deposit to form the resultinglow soluble amphoteric coating complex. If the metal surface on whichthis amphoteric coating complex is deposited is then made cathode in anelectrolyte containing sodium chloride, sodium sulphate or similarionizable sodium or potassium salt and a sufficient current is passed todecompose some of the sodium salt, for example, sodium hydroxide will bedischarged at the interface of the cathode and the amphoteric coatingcomplex. As a result soluble sodium plumbite will be formed at thecathode interface somewhat as follows:

This latter compound dissociates into lead ions only to a very slightextent. However, as these lead ions are discharged by depositing lead onthe cathode a few more ions are liberated immediately. The reaction willcontinue, therefore, until all the plumbite has been used up. Withinreasonably wide limits of cathode current density it has been found byexperiment that firmly bonded electrolytic deposits of metallic lead canbe formed on the cathode at the interface between the cathode and theamphoteric coating complex.

Some of the variables studied are as follows:

Mild steel test panels were coated at room temperature with an initialbasic low soluble coating deposit from synthetic sea water in accordancewith U.S. Patent No. 2,200,469 at 100 milliamperes per square foot.After washing these panels under running tap water various ones were putinto room temperature solutions of lead nitrate having concentrations ofN/ 1, N/2, N/lO and N/100 for varying lengths of time from 1 minute to48 hours. Without any correction the pH values of these solutions variedfrom 3.8, 4.3, 4.7 up to 5.2. After 60 minutes of conversion reactionthe gain in weight of one set of panels was 0.35, 0.28, 0.19 and 0.02gram respectively. Extending the treating times up to 48 hours showed noappreciable gain in weight over a l-hour treatment, and decreasing thetreating times to 1 minute showed a rather rapid drop-01f in conversionreaction rates at times of less than minutes. Also solutions havingconcentrations of less than one normal lead nitrate showed a ratherrapid drop-off in conversion rates. The result is that the study wascontinued with baths of not less than one normal lead nitrate for theroom temperature conversion reaction tests, and with reaction periods of1 hour. It should be emphasized that room temperature conversionreactions are highly desirable for low cost treatment of large tanks orstructures but it is explicitly understood that the examples given inthis application do not constitute a limitation in any way whatever onthe various reaction conditions and variables involved in the hereinlisted amphoterio metal reactions.

After the chemical conversion treatment various panels were thensubjected to an electrolytic reduction treatment in 3.5% sodium chloridesolution having starting pH values of approximately 8.0 and withcathodic current densities varying in substantially geometric ratiosfrom 12.5 milliamperes per square foot up to 12.5 amperes per squarefoot. Although greater ampere-hours will be required for thickerdeposits, in general, reasonably good deposits of metallic lead wereobtained after the expenditure of a quantity of electricity equal to 2.4to about 4.0

4 ampere hours per square foot of cathodic surface. Some of the bestlead deposits have been obtained in the cathodic current density rangesof 12.5 to 800 milliamperes per square foot although some metallic leadwas deposited on each panel up to values of 12.5 amperes per I squarefoot.

Tests were also made when the lead nitrate conversion baths werecorrected with dilute nitric acid to pH values down to 2.0approximately. Improvements in the final lead deposit were obtained asthe pH values of the baths were decreased to 2.0 and somewhat lesstreeing was obtained. However, the lead coatings made without any pHcorrection of the lead nitrate baths were also excellent.

Similar panel tests were run when using N/l lead acetate solutions atroom temperature. The results obtained were substantially like thoseobtained from the lead nitrate conversion baths, except that theconversion reaction rates were somewhat longer for the uncorrected leadacetate solutions having pH values of 5.8 approximately.

Magnesium hydroxide coatings which were electrolytically deposited onthe clean steel panels from magnesium chloride baths and then subjectedto a chemical conversion treatment and electrolytic reduction treatmentsas discussed above gave quite similar results. It is desired that thisapplication include the use of any suitable lead salt for the formationof an amphoteric coating complex as defined in this application.

Zinc coatings Dense well bonded metallic zinc coatings have beendeposited on steel surfaces under the following conditions:

Mild steel test panels were coated with an initial basic coating depositin accordance with the procedure outlined under lead coatings. Somepanels were also coated with an initial basic coating deposit ofmagnesium hydroxide from a solution of magnesium chloride as previouslyoutlined. These various panels were then washed under running tap waterand put in uncorrected pH 5.5 and corrected pH 5.0 solutions of N/l zincchloride. These chemical conversion treatments were varied from 1 hourto 48 hours of treatment in room temperature solutions. As was found forthe lead nitrate treated panels the 1-hour conversion treatments wereessentially as complete as the 48-hour conversion treatments. However,when the temperature of the zinc chloride bath was increased from roomtemperature to 75 C. the chemical reaction time was decreased from 1hour to approxi mately 5 minutes for a similar deposit of the lowsoluble zinc amphoteric coating complex.

The electrolytic reduction treatment of these panels was then made in3.5% sodium chloride solutions at current densities varying frommilliamperes per square foot up to 2.0 amperes per square foot. Althoughvarious electrolytic reduction times have been studied, the reductiontimes were usually varied inversely as the cathodic current densityuntil each panel had received a quantity of electricity at least equalto 2.4 ampere-hours. Although greater ampere-hours will be needed forthicker amphoteric coating complexes, good deposits of metallic zincwere obtained on each of the panels of this zinc series.

When zinc sulphate conversion baths were substituted for the zincchloride conversion baths, essentially the same conditions were found togive useful metallic zinc coatings. It is desired that this applicationinclude the use of any suitable zinc salt for the formation of anamphoteric coatingcomplex as defined in this application.

Tin coatings The procedures set up for lead and zinc were usedessentially with little modification for the formation and treatment ofthe amphoteric coating complex depositions formed from tin salts. Theintial basic coatings were formed by the procedures outlined under leadcoatings.

The chief difference in the procedure was the formation of theamphoteric coating complex depositions.

When using N/l tin chloride solutions the low pH of approximately 1.0resulting from hydrolysis of the tin chloride dissolved the initialbasic coating in about 15 seconds. As a result conversion treatmentsolutions of N/ were used. These N/10 solutions of tin chloride with pHvalues of approximately 2.1 gave good amphoteric coating complexreaction products in 1- and 2-minute treatments of the panels at roomtemperature. Good firmly bonded metallic depositions of tin were thenobtained when these panels were treated at room temperature in 3.5%sodium chloride at the ampere-hours and cathodic current densities usedfor zinc and lead depositions. Well bonded deposits of tin were obtainedin the 3.5% sodium chloride solutions at cathodic current densities of100 to 800 milliamperes per square foot. It is desired that thisapplication include the use of any suitable tin salt for the formationof an amphoteric coating complex as defined in this application.

General One of the outstanding advantages of the use of an amphotericcoating complex to electrolyze into a metallic coating is the inherentfine grain deposits which have been obtained and to the exceptionaluniformity of dep osition. Probably one of the chief reasons why suchfine grain deposits have been obtained over such a wide range ofcathodic current densities is due to the fact that the metal ionconcentration at all times is rather low. As these metal ions aredischarged and plated out on the cathode other ions are immediatelyliberated but only in limited amounts. In other words these amphotericcoating complexes act in a way similar to the metal cyanides in theordinary electroplating baths. Another possible reason for the finegrain metal deposits which are obtained by these processes is that thecathode polarization is greater than that obtained in the ordinaryplating baths. This increased polarization causes a decreased crystalsize of the metal deposit, greater throwing power of the bath and adecrease in treeing of the deposit.

In regard to the two latter advantages the use of these lead, zinc ortin amphoteric coating complexes has greatly increased the throwingpower and decreased the treeing of the resulting electrodeposits whenthey are compared to the electroplating baths and deposits produced bythe ordinary aqueous processes. However, at the higher current densitiestested a slight amount of treeing has been observed. This is probablydue to an excessive formation of the soluble amphoteric metal salt atthe cathode interface and its blowing out through the amphoteric coatingto the electrolyte interface where it is electrolytically reduced,thereby forming a conducting chain of metallic crystal trees. That theabove comments on the formation of crystal trees are probably correct isindicated by a microscopic examination of a partially electrolyzedamphoteric coating complex on a panel. At the higher current densitiestested up to 12.5 amperes per square foot small volcanoes indicating ablow-out action have been found in the vicinity of crystal trees. Inregard to the electrolytes suitable for the reduction operations it isintended that these electrolytes be one or more of the soluble salts ofthe alkali metals. In regard to the current densities used for thereduction operations these current densities should be within thefollowing limits: (a) the minimum current density should be a cathodiccurrent density of suflicient magnitude in an electrolyte which willliberate a hydroxide of the alkali metals at a cathode therebysolubilizing the amphoteric metal component of the amphoteric coatingcomplex at the cathode interface, and (b) the maximum current densityshould be one below which the electrodeposited metal is depositedessentially as a spongy arboreal or burnt deposit.

It is intended that this application cover the use of any of theamphoteric metal salts which can be used to produce a low solubleamphoteric coating complex by any of the methods herein disclosed andwhich can then be cathodically reduced to a useful metallicelectroplated deposit.

While preferred embodiments of the invention are herein disclosed by wayof example, it is understood that the invention is not limited withrespect to the precise mode of applying the initial basic coatingdeposit as an amphoteric coating complex or as an initial basic coatingdeposit which can be treated with a soluble salt of an amphoteric metalto form an amphoteric coating complex, but it is broadly inclusive ofany and all equivalents, both of procedure and constituent substancessuch as fall within the scope of the appended claims.

Related plating methods are disclosed in my copending application SerialNo. 842,021.

I claim:

1. The process of electrolytically depositing an amphoteric metal,selected from the group consisting of lead, tin and zinc, on a ferrousmetal structure which comprises coating said structure with a firmlyadherent amphoteric coating complex of low solubility containing themetal to be deposited, and then subjecting said coated structure to acathodic current density of sufficient magnitude in an electrolytecontaining a soluble alkali metal salt which will liberate a hydroxideof the alkali metal atthe cathode thereby solubilizing the amphotericmetal component of the amphoteric coating complex at the cathodeinterface while at the same time electrodepositing on the cathode theselected amphoteric metal from the solubilized product.

2. The process of electrolytically depositing lead on a ferrous metalstructure which comprises coating said structure with a firmly adherentamphoteric coating complex of low solubility containing lead, and thensubjecting said coated structure to a cathodic current density ofsufficient magnitude in an electrolyte containing a soluble alkali metalsalt which will liberate a hydroxide of the alkali metal at the cathodethereby solubilizing the lead content of the amphoteric coating complexat the cathode interface while at the same time electrodepositing leadon the cathode from the solubilized product.

3. The process of electrolytically depositing zinc on a ferrous metalstructure which comprises coating said structure with a firmly adherentamphoteric coating complex of low solubility containing zinc, and thensubjecting said coated structure to a cathodic current density ofsufiicient magnitude in an electrolyte cintaining a soluble alkali metalsalts which will liberate a hydroxide of the alkali metal at the cathodethereby solubilizing the zinc content of the amphoteric coating complexat the cathode interface while at the same time electrodepositing zincon the cathode from the solubilized product.

4.- The process of electrolytically depositing tin on a ferrous metalstructure which comprises coating said structure with a firmly adherentamphoteric coating complex of low solubility containing tin, and thensubjecting said coated structure to a cathodic current density ofsufficient magnitude in an electrolyte containing a soluble alkali metalsalt which will liberate a hydroxide of the alkali metal at the cathodethereby solubilizing the tin content of the amphoteric coating complexat the cathode interface while at the same time electrodepositing tin onthe cathode from the solubilized product.

5. The process of electrolytically depositing an amphoteric metal,selected from the group consisting of lead, tin and zinc, on a ferrousmetal structure which comprises coating said structure with a firmlyadherent amphoteric coating complex of low solubility containing themetal to be deposited, and then subjecting said coated structure to acathodic current density of suflicient magnitude in an electrolytecontaining a soluble sodium salt which will liberate sodium hydroxide atthe cathode 7 thereby solubilizingthe amphoteric metal component of theamphoteric coating complex at the cathode interface While at the sametime electrolytically depositing on the cathode surface the selectedamphoteric metal from the solubilized product.

6. The process of electrolytically depositing an amphoteri metal,selected from the group consisting of lead, tin and Zinc, on a ferrousmetal structure which comprises coating said structure with a firmlyadherent amphoteric coating complex of low solubility containing themetal to be deposited, and then subjecting said coated structure to acathodic current density of sufficient magnitude in an electrolytecontaining a soluble potassium salt which will liberate potassiumhydroxide at the cathode '8 thereby solubilizing the amphoteric metalcomponent of the amphoteric coating complex at the cathode interfacewhile at the same time electrolytically depositing on the cathodesurface the selected amphoteric metal from the solubilized product.

References Cited in the file of this patent UNITED STATES PATENTS HynerDec. 14, 1948

1. THE PROCESS OF ELECTRLYTICALLY DEPOSITING AN AMPHOTERIC METAL,SELECTED FROM THE GROUP CONSISTING OF LEAD, TIN AND ZINC, ON A FERROUSMETAL STRUCTURE WHICH COMPRISES COATING SAID STRUCTURE WITH A FIRMLYADHERENT AMPHOTERIC COATING COMPLEX OF LOW SOLUBILITY CONTAINING THEMETAL TO BE DEPOSITED, AND THEN SUBJECTING SAID COATED STRUCTURE TO ACATHODIC CURRENT DENSITY OF SUFFICIENT MAG NITUDE IN AN ELECTRLYTECONTAINING A SOLUBLE ALKALI METAL SALT WHICH WILL LIBERATE A HYDSROXIDEOF THE ALKALI METAL AT THE CATHODE THEREBY SOLUBILIZING THE AMPHOTERICMETAL COMPONENT OF TH AMPHOTERIC COATING COMPLEX AT THE CATHODEINTERFACE WHILE AT THE SAME TIME ELECTRODEPOSITING ON THE CATHODE THESELECTED AMPHOTERIC METAL FROM THE SOLUBILIZED PRODUCT.